1. Field of the Invention
The invention relates to an exhaust gas purification device.
2. Description of the Related Art
For example, Unexamined Japanese Patent Publication No. 9-53442 discloses an exhaust gas purification device, which comprises an absorbent arranged in an exhaust passage of an engine for absorbing NO.sub.x when an air-fuel ratio of an exhaust gas flowing thereinto is lean, the absorbent discharging NO.sub.x absorbed therein when a concentration of an oxygen in the exhaust gas decreases. The absorbent is used in an engine which discharges an exhaust gas, the air-fuel ratio of which is lean in the major range of the engine operation.
As mentioned above, the absorbent discharges NO.sub.x absorbed therein when the concentration of the oxygen in the exhaust gas is decreased by HC and CO, and the absorbent purifies NO.sub.x with HC and CO. Further, the purification device comprises a trapping filter arranged in the exhaust passage upstream of the absorbent for trapping exhaust particulates.
The absorbent may also absorb SO.sub.x in the exhaust gas. In this case, the capacity of the absorbent to absorb NO.sub.x therein is decreased. However, SO.sub.x may absorb on the particulates trapped in the trapping filter. Thus, in the above purification device, SO.sub.x does not flow into the absorbent. Therefore, the trapping filter can maintain the capacity of the absorbent to absorb NO.sub.x therein.
The particulates may clog the trapping filter. In this case, the trapping filter restricts the flow of the exhaust as to the downstream of the trapping filter. To prevent this restriction, the trapping filter is regenerated by burning the particulates in the trapping filter when a predetermined period has elapsed.
SO.sub.x adsorbed on the particulates is discharged from the trapping filter when the trapping filter is regenerated. Thus, the capacity of the absorbent to absorb NO.sub.x is decreased when the trapping filter is regenerated.
Therefore, the object of the invention is to prevent the capacity of the absorbent to absorb NO.sub.x from being decreased when the trapping filter is regenerated.